The woman was in her 20s and had a 6-month history of "abnormal toenails," but had no medical problems and no family history of nail disorders, dermatologist Dr. Shari Lipner wrote in the report. But the patient remembered having a fish pedicure "some months prior" to the start of her toenail woes.

The treatment involves dunking the feet and lower legs into an aquarium filled with Garra rufa fish — also known as "doctor fish." In their natural habitats, they subsist on plankton, but in a spa setting, they'll consume human skin cells, Lipner wrote in the report. Fish pedicures, therefore, promise softer, smoother skin.

But for the woman in the case report, those hardworking little exfoliators may have left her with a nail problem, too.

The woman had condition that can cause nail shedding.

Lipner wrote that the woman was diagnosed with onychomadesis. It's phenomenon in which nail production stops completely and usually results in complete shedding of the nail, the report added. It can be caused by trauma, like an injury, or by certain conditions or medications, according to a paper published last year in the journal Cutis.

Lipner wrote that, to her knowledge, this is the first case of onychomadesis linked to a fish pedicure — and, as CNN reported on Tuesday, there's no test that can definitively determine whether the fish caused the condition.

Whether or not the fish were at fault, it's also worth noting a that these pedicures have been linked to another health issue: infections.

Experts say fish pedicures could cause infections.

"Tubs and fish cannot be adequately sanitized between people, and the same fish are typically reused for successive persons," Lipner wrote in the report. "Thus, there are concerns of transmitting infections between people undergoing these pedicures."

That doesn't mean everyone getting a fish pedicure leaves the spa crawling with gnarly pathogens. In fact, the UK government's Health Protection Agency (HPA) published a report on fish pedicure safety in 2011 and concluded that the risk for infection from the treatment is "likely to be very low."

But that risk can't be "completely excluded," the report added. The HPA added that having certain health conditions (diabetes, psoriasis, eczema, athlete's foot or other foot infections, blood-borne viruses like hepatitis and HIV, bleeding disorders, and any immune deficiency) may increase the risk of infection from a fish pedicure. Having open wounds on the legs or feet — or having shaved or waxed the legs during the previous 24 hours — could also ramp up infection risk, the report said.

In some places, fish pedicures are outright banned.

Given all the potential risks, it's not too surprising that at least 10 US states have banned fish pedicures, according to the Centers for Disease Control and Prevention (CDC). The CDC notes that there are a few different reasons these bans are in place, including lack of tub sanitation between customers. But perhaps the most terrifying reason is that sometimes Garra rufa are mistakenly replaced with Chinchin— a fish species that grows teeth and can draw blood.